Human MAbs (huMAbs) with primary response level affinities can now be recovered from filamentous bacteriophage display libraries of "naive" human B cell repertoires. Thus, the goal of generating therapeutic huMAbs without immunization is within reach. What is lacking is a reliable means for completing the affinity maturation process in vitro. To accomplish this we have developed a computer-assisted method for the rational randomization of antibody CDRs called Parsimonious Mutagenesis. This method allows the construction of "scanning" libraries which can "probe" the surface of the antigen one or a few residues at a time with a wide selection of amino acid side chains to search out and identify new high- affinity contacts. We will use the method to probe the upper limits of affinity of a single chain version (scFv) of a murine MAb for its antigen, human complement factor C5a, an important therapeutic target. We will also use the method to humanize this scFv. We will then select additional scFv's against important therapeutic targets from a phage display library of a naive human B cell repertoire, and use the method to mature their affinities in vitro.